Material selection for a high-speed or high-frequency PCB often focuses on dielectric constant (Dk) and dissipation factor (Df). However, the copper conductor also contributes to total transmission loss.
One important but sometimes overlooked parameter is copper foil roughness, especially at the interface between the copper conductor and the PCB dielectric material.
As signal frequency and data rate increase, copper surface characteristics can have a greater influence on conductor loss, insertion loss, phase response and signal propagation. Copper foil profile should therefore be considered together with laminate properties, stack-up design and PCB manufacturing processes.
What Is Copper Foil Roughness?
The copper used in PCB laminates is not perfectly smooth. A certain amount of surface texture is necessary to provide mechanical adhesion between the copper foil and dielectric material. During multilayer PCB manufacturing, additional surface treatment may also be applied to improve bonding between copper and prepreg.
The relevant roughness for high-frequency performance is mainly the copper surface at the copper-to-dielectric interface. Rogers Corporation reports that this interface can influence insertion loss, phase response and propagation speed. It can also affect the Dk value extracted from measurements performed on an actual circuit.
Why Does Copper Roughness Increase Conductor Loss?
At direct current or relatively low frequencies, electrical current uses more of the conductor’s cross-sectional area. As frequency increases, current becomes concentrated closer to the surface of the conductor. This phenomenon is known as the skin effect.
The effective depth through which most of the current flows becomes smaller as frequency rises. When the copper surface is rough, the current encounters a more complex copper-to-dielectric interface. This increases effective conductor resistance and can contribute to higher conductor loss.
Rogers explains that the influence becomes more significant when skin depth is small in relation to the roughness profile. This is why copper roughness receives greater attention in microwave, millimetre-wave and high-speed digital applications.
Copper Loss Is Only One Part of PCB Insertion Loss
Total insertion loss is not caused by a single factor. It can include:
Dielectric loss
Conductor loss
Radiation loss
Losses associated with vias, connectors and other discontinuities
The dielectric material’s Df is closely related to dielectric loss, while copper conductivity, trace geometry and surface roughness contribute to conductor loss. Selecting a low-Df laminate can reduce dielectric loss, but the expected improvement may be partly limited if the PCB uses very rough copper or an unsuitable bonding treatment.
A reliable low-loss PCB requires the dielectric material and copper construction to be evaluated together.
How Copper Roughness Affects Insertion Loss
Insertion loss describes how much signal power is lost as a signal travels through a transmission path. Rougher copper generally produces greater conductor loss than smoother copper, and the difference becomes more relevant as frequency increases.
Rogers notes that copper roughness can have a larger influence on insertion loss when the substrate is thin. For long high-speed transmission channels, even a relatively small increase in loss per unit length can become important across the complete signal path.
This consideration is relevant for:
High-speed backplanes
Server and data-storage equipment
High-speed communication systems
RF and microwave modules
Radar and automotive sensing systems
5G and millimetre-wave equipment
Precision test and measurement products
Can Copper Roughness Affect Dk and Propagation Speed?
Copper roughness does not change the chemical composition of the dielectric laminate. However, it can affect how a signal behaves in the completed transmission line.
Rogers reports that rough copper can slow electromagnetic wave propagation. As a result, the effective or circuit-perceived Dk extracted from measurements may appear higher than the actual Dk of the dielectric material.
Signal propagation delay
Phase response
Effective Dk
Impedance modelling
Insertion-loss prediction
For high-frequency simulations, using only a laminate’s nominal Dk and Df values may not fully represent the manufactured circuit. Copper foil type, surface profile and fabrication process should also be considered when accurate modelling is required.
Standard Copper, VLP and HVLP Copper
Standard electrodeposited copper generally has a more pronounced surface profile. Low-profile copper options, commonly described as VLP or HVLP copper, are designed to provide a smoother interface while maintaining adequate bonding performance.
Using low-profile copper can help reduce conductor loss, but the designation alone does not provide all the information required to predict circuit performance. Engineers should also consider:
Copper roughness data
Copper thickness
Laminate construction
Dielectric thickness
Transmission-line geometry
Bonding treatment
Required operating frequency
Target insertion-loss performance
Different foil suppliers and laminate systems may use different constructions. The exact copper option should therefore be confirmed with the PCB material supplier and manufacturer.
Why Not Always Use the Smoothest Copper?
Copper foil must provide both electrical performance and reliable adhesion to the dielectric material. A rougher surface can improve mechanical bonding, but it may increase conductor loss. A smoother surface can reduce conductor loss, but the laminate and bonding system must still provide sufficient peel strength and manufacturing reliability.
Signal-loss requirements
Copper-to-dielectric adhesion
Thermal reliability
Material availability
Manufacturability
Project cost
Modern laminate and copper-treatment technologies aim to balance lower conductor loss with suitable bonding performance.
How PCB Manufacturing Can Affect Copper Roughness
Copper roughness is influenced not only by the original copper foil but also by the PCB manufacturing process. During multilayer production, the copper surface may be treated before lamination to improve adhesion to the prepreg. The selected oxide or oxide-alternative treatment can affect the final interface seen by the signal.
IPC-published research involving AT&S and Vienna University of Technology identifies copper foil quality and oxide-replacement treatment as important factors in controlling copper-related loss.
Approved laminate and copper foil combinations
Inner-layer bonding treatment
Lamination parameters
Copper thickness
Trace width and geometry
Etching consistency
Final impedance and electrical performance
Substituting a laminate or copper foil without considering these factors may change PCB performance, even if the replacement has similar nominal Dk and Df values.
What Should Engineers Include in the PCB Specification?
For high-speed or high-frequency projects, “low-loss material” alone may not be a complete requirement. Useful fabrication information may include:
Approved laminate brand and material grade
Copper foil type or profile requirement
Required copper thickness
Controlled-impedance values and tolerances
Stack-up and dielectric thickness
Maximum insertion-loss requirement, if applicable
Operating frequency or data rate
Test-coupon requirements
Acceptable alternative materials
Required validation or test method
If the original material is difficult to obtain, any proposed alternative should be evaluated based on the complete material construction rather than Dk and Df alone.
What Should Purchasing and EMS Companies Confirm?
Purchasing teams and EMS providers do not necessarily need to define every copper parameter themselves. Before approving a material substitution, it is useful to ask:
Does the alternative use a comparable copper foil profile?
Will the same inner-layer bonding treatment be used?
Could the substitution affect insertion loss or impedance?
Is the proposed construction supported by the laminate supplier?
Can impedance coupons or loss testing be provided when required?
These questions help reduce the risk of approving a material based only on a similar Dk value or general “low-loss” classification.
How Can Copper-Roughness Effects Be Verified?
The appropriate verification method depends on the project’s frequency range, channel length and performance requirements.
Review of laminate and copper foil data
Controlled-impedance testing
Microsection inspection
Insertion-loss measurements
Time-domain or frequency-domain testing
Comparison between test coupons and simulation models
For demanding applications, the measurement method and acceptance limits should be agreed upon before production.
Conclusion
Copper foil roughness is an important manufacturing and material consideration for high-speed and high-frequency PCBs. As frequency increases, the skin effect concentrates current near the copper surface. A rougher copper-to-dielectric interface can increase conductor resistance and insertion loss. It may also influence phase response, propagation speed and the effective Dk observed in circuit measurements.
Low-profile copper can help reduce these effects, but electrical performance must be balanced with copper-to-dielectric adhesion and manufacturing reliability. The best approach is to evaluate the laminate, copper foil, stack-up, bonding treatment and performance requirements as one complete PCB construction.
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